LP#1889251: make call number affix sort keys read-only

LP#1889251: make call number affix sort keys read-only

This patch makes the call number prefix and suffix label sort
key fields read-only on their respective Server Administration
pages, as any values that a user would try to set for them
would get overwritten by a database trigger.

To test
-------
[1] Apply the patch.
[2] Go to Server Administration -> Call Number Prefixes (or Suffixes)
    and attempt create and edit new affixes.
[3] Verify that the label sort key is a read-only display field on the
    edit modal.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>